A superconducting accelerator cavity is formed by joining a plurality of members that are placed one after another in an axial direction. This joining is conventionally performed from outside by penetration welding by electron beam welding or laser welding in a vacuum atmosphere because a vacuum atmosphere has low contamination caused by impurities.
In the case in which penetration welding from outside is employed in this way, because unexpected irregularities are sometimes formed in terms of the welded states inside the superconducting accelerator cavity, post-processing therefor is time consuming. In addition, because a driving system that drives the superconducting accelerator cavity, an electron gun, and so forth are placed in the vacuum atmosphere, the volume of vacuum container is increased, which increases the size of the equipment and makes it expensive. Because it takes time to establish the vacuum atmosphere due to the large vacuum container volume, the operation time is increased, which increases the operating costs.
Therefore, there is a demand for reducing internal post-processing by performing welding from inside a superconducting accelerator cavity.
As a technique for doing so, it has been proposed, for example, to perform joining in a superconducting accelerator cavity from the inside by employing laser welding in an argon atmosphere, as disclosed in Patent Literature 1.
However, sufficient performance may not be achieved with the technique in which the laser welding is performed in an argon atmosphere.